PLASMA CORTICOSTEROIDS AND THE EFFECT OF ADRENOCORTICOTROPHIN IN A MACROPODID MARSUPIAL (SETONIX BRACHYURUS, QUOY & GAIMARD)

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Abstract
The total corticosteroid concentrations in the peripheral plasma of unanaesthetized, undisturbed quokkas were 0·75 ± 0·10 (s.e.m.) and 0·93 ± 0·14 μg/100 ml in male and female quokkas respectively. Repeated sampling for periods of 36–49 h disclosed irregular fluctuations over the range 0·4–5·0 μg/100 ml, but no evidence for a regular periodicity. The major corticosteroid was usually cortisol but corticosterone contributed 25–50% of the total unstimulated corticosteroid concentration. Relatively minor concentrations of 11-deoxycortisol were detected. Constant-rate i.v. infusion of ACTH caused a significant increase in the concentration of total corticosteroids in the plasma; this increase was detectable at a dose of 0·05 i.u. ACTH/kg/h, and rose to approximately 15 times the control value at a dose of 2·0 i.u./kg/h. This increase was due mainly to a change in the concentration of cortisol. Synthetic (β1–24) and porcine ACTH were equipotent. The sensitivity of the quokka to ACTH was approximately one-tenth that of another marsupial (Trichosurus vulpecula) or 1/160 that of man. Moderate disturbance increased the concentration of corticosteroids in the plasma to the same level as that caused by infusion of 0·1 i.u. ACTH/kg/h, the increase being mainly in the cortisol fraction. High-affinity binding of cortisol and corticosterone by plasma proteins was demonstrated. The maximum binding capacities for cortisol were 3·89 ± 0·5 and 3·02 ± 0·6 μg/100 ml in female and male quokkas respectively. The mean association constant was 3·2 × 1081/mol at 4 °C and 5·5 × 1071/mol at 36 °C. The maximum binding capacity for corticosterone was approximately one-third that of cortisol.